The present invention relates to a flyback converter with synchronous rectifying function, especially to a flyback converter with synchronous rectifying function to enhance efficient and reduce power consumption at empty load condition.
The flyback converter with rectifying device is extensively used for power supplies. FIG. 1 shows the circuit diagram of a prior art flyback converter with rectifying device. When the gate of the thyristor M1 is triggered and the thyristor M1 is turned on, the positive pole of the auxiliary winding N1 on the secondary side of the transformer T is high. The high voltage turns on a controllable unit Si through a resistor R2. Therefore, a MOS transistor SR is turned off.
On the contrary, when the gate of the thyristor M1 is turned off, the negative pole of the middle auxiliary winding N2 on the secondary side of the transformer T is high. The high voltage turns on the MOS transistor SR through a diode D1 and a resistor R1.
However, the MOS transistor SR has considerable inversed current when it is turned off. The synchronous rectifying effect of the rectifying device is degraded by the extra power consumption. Moreover, the MOS transistor SR has power consumption at empty load.
FIG. 2 shows the circuit diagram of another prior art rectifying device for flyback converter. When the gate of the thyristor M1 is turned off, the negative pole of the second auxiliary winding N1 on the secondary side of a transformer T is high. The high voltage turns on the MOS transistor SR through a diode D1 and a resistor R1. At this time, a current transformer TA senses a reversed output current and the negative pole on the secondary side of the current transformer TA is high. The high voltage turns a controllable unit S1 through a resistor R2. Therefore, a MOS transistor SR is turned off.
However, the MOS transistor SR still has considerable power consumption at empty load.
It is an object of the present invention to provide a flyback converter with synchronous rectifying function and enhanced efficiency. The problem of large reversed current in MOS transistor is solved and the MOS transistor is turned off at low load condition to reduce power consumption.
It is another object of the present invention to provide a flyback converter with synchronous rectifying function. At switching instance, a small reversed current is present at a secondary winding of a transformer to provide resonance energy for an inductor of a primary winding and a parasite. A thyristor can be turned on at low voltage to reduce power consumption for turning on the thyristor.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: